1. Field of the Invention
The present invention relates to a semiconductor integrated circuit including a resistor circuit made of polycrystalline silicon.
In a semiconductor integrated circuit, the following two resistors have been mainly used: a diffusion resistor, which is made of single crystalline silicon to which an impurity of an opposite conductivity to that of a semiconductor substrate is diffused, and a polycrystalline silicon resistor, which is made of polycrystalline silicon to which an impurity is diffused. Between the two, a polycrystalline silicon resistor has been widely used for a semiconductor integrated circuit because of its advantages, for example, that leak current is small since an insulating film surrounds the resistor, and that high resistance can be obtained owing to defects that exist at grain boundaries.
2. Description of the Related Art
FIGS. 2A and 2B show a conventional polycrystalline silicon resistor circuit. (For example, refer to JP 2002-76281 A.) FIG. 2A is a schematic plan view of the resistor circuit, and FIG. 2B is a sectional view which is taken along the line A-A′ of FIG. 2A. The polycrystalline silicon resistor is formed by: implanting a p-type or N-type impurity to a polycrystalline silicon thin film deposited on an insulating film with an LPCVD method or the like; and processing the resultant into a resistor shape with a photolithography technique. Impurity implantation is carried out in order to determine the resistivity of the polycrystalline silicon resistor. Concentration of the P-type or N-type impurity ranges from 1×1017/cm3 to 1×1020/cm3 in accordance with the desired resistance.
Further, a contact hole and a metal wiring are disposed in each of terminals on both sides of a resistor to take out the potential thereof. To obtain a satisfactory ohmic contact between the polycrystalline silicon film and the metal wiring at the terminal, an impurity at a high concentration of 1×1020/cm3 or more is diffused into a part of the polycrystalline silicon film, which forms the resistor terminal.
In the case of structuring a resistor circuit shown in FIG. 3, the resistor using polycrystalline silicon is, therefore, formed of a polycrystalline silicon film 3 which is formed on an insulating film 2 on a semiconductor substrate 1 and which is composed of a low concentration impurity region 4 and high concentration impurity regions 5, as shown in the schematic plan view of FIG. 2A and the schematic sectional view of FIG. 2B. A potential of a terminal D (104) is taken out from a terminal A (101) through a metal wiring 7 via contact holes 6 formed in the second insulating film 9 and provided on the high concentration impurity regions 5.
Further, in FIG. 2A, between two metal wirings connected to the respective contact holes on both the sides of the resistor, one metal electrode is formed so as to cover the low concentration impurity region 4 in the polycrystalline silicon film 3. There are two reasons for the structure.
The first reason is to obtain stability of the polycrystalline silicon resistor. Since the polycrystalline silicon is a semiconductor, when a wiring or electrode is formed thereon, depletion or accumulation occurs in the polycrystalline silicon due to a relative relationship between the potential of the wiring or electrode and the potential of the polycrystalline silicon resistor, which varies a resistance value of the resistor. Specifically, as regards the polycrystalline silicon to which P-type impurity is diffused, the P-type polycrystalline silicon becomes depleted, which leads to a high resistance when the wiring or electrode having a higher potential than that of the polycrystalline silicon resistor exists immediately above the polycrystalline silicon. In the opposite potential relationship, the resistance lowers due to accumulation. In order to avoid such variation in resistance, a wiring having a potential close to that of the polycrystalline silicon is intentionally formed on the polycrystalline silicon, thereby keeping the constant resistance. One example of this is the plan view of FIG. 2A, where one of the electrodes of the polycrystalline silicon resistor is extended to the resistor to fix the potential.
The phenomenon described above naturally depends on not only the wiring above the polycrystalline silicon but also the state thereunder. That is, the resistance varies in accordance with the relative relationship in potential between the polycrystalline silicon resistor and a semiconductor substrate under the polycrystalline silicon resistor. A means for stabilizing the potential has been hence known in which a diffusion region or the like is intentionally formed under the polycrystalline silicon resistor in the same manner as the upper metal wiring although the diffusion region is not shown in the figure.
The second reason is to prevent hydrogen, which affects the polycrystalline silicon resistance, from diffusing to the polycrystalline silicon in a semiconductor manufacturing process. Polycrystalline silicon is composed of a grain with relatively high crystallinity and a grain boundary between grains having low crystallinity, that is, high trap levels. The resistance of a polycrystalline silicon resistor is almost determined by carriers, electrons or holes, which are trapped by a large number of levels that exist at the grain boundary. When hydrogen with a high diffusion coefficient is generated in the semiconductor manufacturing process, the generated hydrogen easily reaches the polycrystalline silicon resistor and is trapped by the level, which varies the resistance.
Examples of such hydrogen generating processes include a step of sintering in a hydrogen atmosphere after the metal electrode formation and a step of forming a plasma nitride film with the use of an ammonia gas.
Accordingly, in the resistor circuit as shown in FIGS. 2A and 2B, the variation in resistance of the polycrystalline silicon due to the above mentioned hydrogen diffusion can be suppressed by covering the polycrystalline silicon resistor with a metal wiring.
However, a method for stabilization of the polycrystalline silicon resistance of the above-described resistor circuit has the following problem. The problem is that: the metal wiring should be arranged to sufficiently cover the resistor in consideration for an alignment shift between the metal wiring and the polycrystalline silicon resistor and for preventing effect against the above-mentioned hydrogen diffusion when the metal electrode is disposed over the polycrystalline silicon resistor. As shown in FIG. 2A, a plurality of sets each including the polycrystalline silicon resistor and the metal wiring are arranged for resistors 201, 202, and 203. The area of the resistor circuit is not determined by a pitch/space of the polycrystalline silicon resistor but by a pitch/space of the metal wiring which covers the polycrystalline silicon resistor with a predetermined overlap amount and which has a larger processing dimension than that of the polycrystalline silicon resistor. This hinders the reduction in area of the resistor circuit.